A plug is a type of downhole tool that is designed to isolate two (e.g., axially-offset) portions of a wellbore. More particularly, once the plug is set in the wellbore, the plug isolates upper and lower portions of the wellbore while the upper portion is tested, cemented, stimulated, produced, injected into, or the like. The plug may be a bridge plug or a frac plug.
The plug includes one or more slips that are configured to expand radially-outward and into contact with an outer tubular (e.g., a casing) or the wall of the wellbore when the plug is set, to anchor the plug in place. The outer radial surfaces of the slips typically include a plurality of teeth that are configured to “bite” into the outer tubular or the wall of the wellbore to improve the strength of the anchor.
The slips may be made from metal, such as cast iron, or a composite (e.g., fiber-reinforced glass or other such) material. In the latter case, the composite material makes the plug easier to mill out of the wellbore when its use is complete. However, composite materials generally cannot bite into a metal casing (or any other type of surrounding tubular) with sufficient force to resist movement under high pressure. Accordingly, “buttons” made of a harder material, such as carbide or ceramic, are sometimes bonded to the composite slips, which provide the point of contact with the casing. These buttons, however, are prone to detaching from the slips in the well. Further, the size of the buttons is generally constrained, because the buttons can be difficult to mill. The buttons also add to the cost of the plug and complicate the assembly.